Binder for lithium ion secondary battery positive electrodes

ABSTRACT

Provided is a binder for lithium ion secondary battery positive electrodes, which is capable of providing a positive electrode for lithium ion secondary batteries with excellent high-rate discharge characteristics. A binder for lithium ion secondary battery positive electrodes, which contains a binder resin and 500 ppm or less of an oil-soluble radical initiator.

TECHNICAL FIELD

The present invention relates to a binder for lithium ion secondarybattery positive electrodes which is capable of imparting excellenthigh-rate discharge characteristics to a positive electrode for lithiumion secondary batteries, a positive electrode for lithium ion secondarybatteries including the binder, and a lithium ion secondary batteryprovided with the positive electrode.

BACKGROUND ART

In recent years, with the prevalence of mobile electronic equipment suchas a notebook personal computer, a smartphone, portable game equipment,and a PDA, in order to make these kinds of equipment lighter in weightand enable the equipment to be used for a long period of time, there hasbeen a demand for attaining reduction in size of a secondary batteryused as a power source and highly enhanced energy density thereof.

In particular, there has been an increase in the utilization of asecondary battery as a power source for vehicles such as anelectrically-powered car, an electric motorcycle, and an electricvehicle. Such a secondary battery used also as a power source forvehicles is required to operate even over a wide temperature range aswell as being required to have highly enhanced energy density.

As the secondary battery, a nickel-cadmium battery, a nickel-hydrogenbattery, and the like have hitherto been the mainstream, but there is atendency for a lithium ion secondary battery to be increasingly used.

Usually, a binder for electrodes (hereinafter, sometimes referred tosimply as a binder) is dissolved/dispersed in a solvent/dispersionmedium to prepare a binder solution/binder dispersion, thesolution/dispersion is mixed with an active material and a conductiveadditive to prepare mixture slurry for electrodes (hereinafter,sometimes referred to simply as slurry), the slurry is applied onto acurrent collector, the solvent/dispersion medium is removed by a methodsuch as drying, and the active material, the conductive additive, andthe current collector are bound to one another to produce an electrodeof the secondary battery.

Although a fluorine-based resin such as polyvinylidene difluoride (PVdF)has hitherto been used as the binder for electrodes, there is a problemthat use of an organic solvent causes an increase in the environmentalload because the fluorine-based resin is in a state of being dissolvedin an organic solvent to be used in a solution state.

As such, there have been proposed binder compositions for lithium ionsecondary batteries such as carboxymethyl cellulose (CMC) and astyrene-butadiene rubber latex (SBR) and a crosslinked compound ofpolyacrylic acid substituted with an alkali cation and polyvinylalcohol, which can be dissolved or dispersed in water, are inexpensive,and give little environmental loads (Patent Document 1 and PatentDocument 2).

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent Laid-open Publication No. 2010-146871

Patent Document 2: Japanese Patent Laid-open Publication No. 2008-204829

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, the present inventors conducted studies, whereupon they foundout that there are cases where the electrode is made larger in internalresistance, for example, in the case where the binder composition forlithium ion secondary batteries disclosed in Patent Document 1 or PatentDocument 2 is used and the like.

The present inventors conducted further studies, whereupon they foundout that an oil-soluble radical initiator, which was used in producing abinder resin contained in a binder composition by polymerization,remains in the binder composition, the oil-soluble radical initiator ora by-product produced when an oil-soluble radical initiator reacts withan electrolytic solution or the like causes a rise in internalresistance, and this causes high-rate discharge characteristics of abattery to be deteriorated.

Under such circumstances, a main object of the present invention is toprovide a binder for lithium ion secondary battery positive electrodeswhich is capable of suppressing a rise in internal resistance of alithium ion secondary battery and imparting excellent high-ratedischarge characteristics, a positive electrode prepared with thebinder, a lithium ion secondary battery prepared with the positiveelectrode, and electrical equipment mounted with the lithium ionsecondary battery.

Means for Solving the Problem

The present inventors made earnest investigations in order to solve theabove-mentioned problems. As a result, the present inventors found outthat, by setting the content of the oil-soluble radical initiator in abinder for lithium ion secondary battery positive electrodes to aprescribed value or less, the electrode is made smaller in internalresistance and a lithium ion secondary battery excellent in high-ratedischarge characteristics can be obtained, and thus, the presentinvention has been completed. That is, the present invention providesthe following embodiments.

Aspect 1.

A binder for lithium ion secondary battery positive electrodes includinga binder resin and an oil-soluble radical initiator in a concentrationof 500 ppm or less.

Aspect 2.

The binder for lithium ion secondary battery positive electrodesdescribed in Aspect 1, wherein the oil-soluble radical initiator is atleast one kind selected from the group consisting of an organicperoxide, an azo compound, and a redox initiator.

Aspect 3.

The binder for lithium ion secondary battery positive electrodesdescribed in Aspect 1 or 2, wherein the binder resin is a suspensionpolymer, an emulsion polymer, a dispersion polymer, or a precipitationpolymer constituted of monomer units.

Aspect 4.

The binder for lithium ion secondary battery positive electrodesdescribed in any one of Aspects 1 to 3, wherein the binder resincontains at least one kind selected from the group consisting ofpoly(meth)acrylic acid, polyoxyethylene, polyvinyl alcohol, astyrene-butadiene rubber latex, polyacrylonitrile, polyethylene,polypropylene, polybutadiene, polytetrafluoroethylene, an ethylene-vinylacetate copolymer, an interpolymer constituted of ethylenicallyunsaturated carboxylic acid-alkali metal neutralized product units andvinyl alcohol units, and an alkyl-modified carboxyl group-containinginterpolymer.

Aspect 5.

The binder for lithium ion secondary battery positive electrodesdescribed in Aspect 4, wherein the alkyl-modified carboxylgroup-containing interpolymer is prepared by copolymerizing a(meth)acrylic acid alkyl ester having an alkyl group with 18 to 24carbon atoms in a proportion of 0.1 to 10 parts by mass with 100 partsby mass of (meth)acrylic acid.

Aspect 6.

A positive electrode for lithium ion secondary batteries including anactive material, a conductive additive, and the binder for lithium ionsecondary battery positive electrodes described in any one of Aspects 1to 5.

Aspect 7.

The positive electrode for lithium ion secondary batteries described inAspect 6, wherein the binder in an amount of 0.5 to 30% by mass relativeto the total mass of the active material, the conductive additive, andthe binder is included.

Aspect 8.

A lithium ion secondary battery being provided with the positiveelectrode for lithium ion secondary batteries described in Aspect 6 or7.

Aspect 9.

Electrical equipment being mounted with the lithium ion secondarybattery described in Aspect 8.

Aspect 10.

A use of a composition including a binder resin and an oil-solubleradical initiator in a concentration of 500 ppm or less for a binder forlithium ion secondary battery positive electrodes.

Aspect 11.

A method of producing a binder for lithium ion secondary batterypositive electrodes including the step of mixing a binder resin and anoil-soluble radical initiator in a concentration of 500 ppm or less.

Advantages of the Invention

According to the present invention, the content of the oil-solubleradical initiator in a binder for lithium ion secondary battery positiveelectrodes is set to a prescribed value or less. On this account, thebinder for lithium ion secondary battery positive electrodes of thepresent invention can make the electrode smaller in internal resistanceand can impart excellent high-rate discharge characteristics to alithium ion secondary battery. Furthermore, according to the presentinvention, there can be provided a positive electrode for lithium ionsecondary batteries prepared with the binder, a lithium ion secondarybattery prepared with the positive electrode, and electrical equipmentmounted with the lithium ion secondary battery. The lithium ionsecondary battery according to the present invention has excellenthigh-rate discharge characteristics as compared with a conventionallithium ion secondary battery, both the technical advancement infunction of a battery and the cost reduction thereof can be achieved,and the range of usage applications can be enlarged.

EMBODIMENTS OF THE INVENTION

Hereinafter, the binder for lithium ion secondary battery positiveelectrodes of the present invention, a positive electrode for lithiumion secondary batteries prepared with the binder, a lithium ionsecondary battery prepared with the positive electrode, and electricalequipment mounted with the lithium ion secondary battery will bedescribed in detail.

<Binder for Lithium Ion Secondary Battery Positive Electrodes>

The binder for lithium ion secondary battery positive electrodes of thepresent invention is characterized as including a binder resin and anoil-soluble radical initiator in a concentration of 500 ppm or less.

In the binder for lithium ion secondary battery positive electrodes ofthe present invention, the oil-soluble radical initiator is an initiatorthat should be used for producing a binder resin by a free radicalpolymerization of monomers. Although the oil-soluble radical initiatoris not particularly restricted, preferred examples thereof include anorganic peroxide, an azo compound, a redox initiator, and the like. Onekind of the oil-soluble radical initiator may be used alone and two ormore kinds thereof may be used in combination. In this connection, inthe present invention, even in the case where two or more kinds of theoil-soluble radical initiator are included in a binder, the content ofthe oil-soluble radical initiator in the binder is 500 ppm or less.

Specific examples of the oil-soluble radical initiator includeα,α′-azoisobutyronitrile, 2,2′-azobis-2,4-dimethylvaleronitrile,2,2′-azobismethyl isobutylate, benzoyl peroxide, lauroyl peroxide,cumene hydroperoxide, tertiary butyl hydroperoxide, and the like. Aboveall, from the viewpoints of ease of handling and being excellent instability, α,α′-azoisobutyronitrile is preferred.

The binder resin is a free radical polymer constituted of monomer unitsand is produced by a free radical polymerization using the oil-solubleradical initiator described above. Specific examples of the binder resininclude a suspension polymer constituted of monomer units, an emulsionpolymer constituted thereof, a dispersion polymer constituted thereof, aprecipitation polymer constituted thereof, and the like.

Examples of the binder resin include poly(meth)acrylic acid,polyoxyethylene, polyvinyl alcohol, a styrene-butadiene rubber latex,polyacrylonitrile, polyethylene, polypropylene, polybutadiene,polytetrafluoroethylene, an ethylene-vinyl acetate copolymer, aninterpolymer constituted of ethylenically unsaturated carboxylicacid-alkali metal neutralized product units and vinyl alcohol units, analkyl-modified carboxyl group-containing interpolymer, and the like. Ofthese, from the viewpoints of easy availability of the material, thebinding property attributed to the affinity with a conductive additive,and the like, an alkyl-modified carboxyl group-containing interpolymeris preferably used. One kind of the binder resin may be used alone andtwo or more kinds thereof may be used in combination.

Hereinafter, the alkyl-modified carboxyl group-containing interpolymerwhich is suitable as the binder resin in the present invention will bedescribed in detail. In the present invention, it is preferred that thealkyl-modified carboxyl group-containing interpolymer be prepared bycopolymerizing a (meth)acrylic acid alkyl ester having an alkyl groupwith 18 to 24 carbon atoms in a proportion of 0.1 to 10 parts by mass orso with 100 parts by mass of (meth)acrylic acid. In this connection, inthe present invention, “(meth)acrylic acid” refers to a general term for“acrylic acid and methacrylic acid” and the same holds true for the casesimilar to this.

In the present invention, examples of the (meth)acrylic acid includeacrylic acid, β-methylacrylic acid, methacrylic acid, and the like, andacrylic acid and methacrylic acid are suitably used. One kind of the(meth)acrylic acid may be used alone and two or more kinds thereof maybe used in combination.

In the present invention, the (meth)acrylic acid alkyl ester having analkyl group with 18 to 24 carbon atoms refers to an ester of(meth)acrylic acid with a higher alcohol having an alkyl group with 18to 24 carbon atoms, and examples thereof include stearyl acrylate,eicosanyl acrylate, behenyl acrylate, tetracosanyl acrylate, stearylmethacrylate, eicosanyl methacrylate, behenyl methacrylate, tetracosanylmethacrylate, and the like. Of these, from the viewpoints of beinginexpensive and easily available and excellent coating properties of abinder composed of the resulting interpolymer and the viewpoint ofbinding strength, stearyl (meth)acrylate, eicosanyl (meth)acrylate,behenyl (meth)acrylate, and tetracosanyl (meth)acrylate are suitablyused. In this connection, as the (meth)acrylic acid alkyl ester havingan alkyl group with 18 to 24 carbon atoms, for example, a commercialproduct such as BLEMMER VMA70 (trade name) available from NOFCORPORATION may be used. One kind of the (meth)acrylic acid alkyl esterhaving an alkyl group with 18 to 24 carbon atoms may be used alone andtwo or more kinds thereof may be used in combination.

In the present invention, with regard to the combination of(meth)acrylic acid and a (meth)acrylic acid alkyl ester having an alkylgroup with 18 to 24 carbon atoms which constitute an alkyl-modifiedcarboxyl group-containing interpolymer, one kind of the (meth)acrylicacid and one kind of the (meth)acrylic acid alkyl ester may be combined,two or more kinds of one of the (meth)acrylic acid and the (meth)acrylicacid alkyl ester may be used to be combined with one kind of the otherthereof, and two or more kinds of one of the (meth)acrylic acid and the(meth)acrylic acid alkyl ester may be used to be combined with two ormore kinds of the other thereof.

In the binder for lithium ion secondary battery positive electrodes ofthe present invention, although the proportion of the (meth)acrylic acidalkyl ester having an alkyl group with 18 to 24 carbon atoms is notparticularly limited, from the viewpoints of preventing peeling of amixture for electrodes and elimination of an active material from thecurrent collector and imparting excellent binding durability againstrepeated charging and discharging, the proportion thereof is preferably0.1 to 10 parts by mass or so and more preferably 0.1 to 5 parts by massor so relative to 100 parts by mass of (meth)acrylic acid. When theproportion of the (meth)acrylic acid alkyl ester having an alkyl groupwith 18 to 24 carbon atoms is less than 0.1 parts by mass relative to100 parts by mass of (meth)acrylic acid, there is a case where thebinding capacity of a binder becomes insufficient because thehydrophobic interaction by the alkyl group becomes weak. On the otherhand, when being more than 10 parts by mass, there is a case where analkyl-modified carboxyl group-containing interpolymer becomes difficultto be uniformly dispersed in a liquid medium such as water describedbelow because the hydrophobicity becomes strong.

In the present invention, in addition to the (meth)acrylic acid and the(meth)acrylic acid alkyl ester having an alkyl group with 18 to 24carbon atoms, a compound having two or more ethylenically unsaturatedgroups may be further copolymerized therewith. Although the compoundhaving two or more ethylenically unsaturated groups is not particularlyrestricted, from the viewpoints of preventing peeling of a mixture forelectrodes and elimination of an active material from the currentcollector and imparting excellent binding durability against repeatedcharging and discharging, a compound with an allyl group as theethylenically unsaturated group is preferred. Furthermore, of these,from the viewpoint of enhancing the binding property with an activematerial, a conductive additive such as carbon fiber, and a currentcollector made of aluminum, copper, or the like, pentaerythritol allylethers such as pentaerythritol diallyl ether, pentaerythritol triallylether, and pentaerythritol tetraallyl ether, diethylene glycol diallylether, polyethylene glycol diallyl ether, polyallylsaccharose, and thelike are further preferred. In this connection, these compounds havingtwo or more ethylenically unsaturated groups may be used alone and twoor more kinds thereof may be used in combination.

In the present invention, in the case of using a compound having two ormore ethylenically unsaturated groups, the proportion thereof ispreferably 0.5 parts by mass or less, more preferably 0.001 to 0.5 partsby mass or so, and further preferably 0.01 to 0.2 parts by mass or sorelative to 100 parts by mass of (meth)acrylic acid. When the proportionof the compound having two or more ethylenically unsaturated groups is0.5 parts by mass or less, slurry containing a binder becomeshomogeneous and there is no fear that the battery performance islowered.

Although the weight average molecular weight of an alkyl-modifiedcarboxyl group-containing interpolymer included in the binder of thepresent invention is not particularly restricted, for example, theweight average molecular weight thereof is 10000 to 10000000 or so. Inthis connection, the weight average molecular weight refers to a valueobtained by being measured by gel permeation chromatography (GPC) usingthe standard polystyrene.

In the present invention, a method of making (meth)acrylic acid, a(meth)acrylic acid alkyl ester having an alkyl group with 18 to 24carbon atoms, and a compound having two or more ethylenicallyunsaturated groups used as necessary undergo a copolymerization toobtain an alkyl-modified carboxyl group-containing interpolymer is notparticularly limited, and a usual method such as a method of stirringthese raw materials in a solvent under an inert gas atmosphere to bemade to undergo a polymerization with the use of an oil-soluble radicalinitiator can be used. The polymerization method is not particularlyrestricted, usual polymerization methods such as emulsionpolymerization, suspension polymerization, dispersion polymerization,solution polymerization, and precipitation polymerization can beadopted, and emulsion polymerization, suspension polymerization,dispersion polymerization, and precipitation polymerization can bepreferably adopted. Examples of the inert gas include nitrogen gas,argon gas, and the like.

Moreover, a solvent used in the copolymerization is not particularlylimited as long as the solvent is one that dissolves (meth)acrylic acid,a (meth)acrylic acid alkyl ester having an alkyl group with 18 to 24carbon atoms, and a compound having two or more ethylenicallyunsaturated groups used as necessary but does not dissolve analkyl-modified carboxyl group-containing interpolymer produced by thecopolymerization and is one that does not inhibit the reaction. Specificexamples of the solvent include chain hydrocarbons such as normalpentane, normal hexane, isohexane, normal heptane, normal octane, andisooctane; alicyclic hydrocarbons such as cyclopentane,methylcyclopentane, cyclohexane, and methylcyclohexane; aromatichydrocarbons such as benzene, toluene, xylene, and chlorobenzene;halogenated hydrocarbons such as ethylene dichloride; esters such asethyl acetate and isopropyl acetate; ketones such as methyl ethyl ketoneand methyl isobutyl ketone, and the like. One kind of the solvent may beused alone and two or more kinds thereof may be used in combination.

The oil-soluble radical initiator used for copolymerization is notparticularly limited as long as the initiator is a oil-soluble radicalinitiator that can be made soluble in oil. Examples thereof includeα,α′-azoisobutyronitrile, 2,2′-azobis-2,4-dimethylvaleronitrile,2,2′-azobismethyl isobutylate, benzoyl peroxide, lauroyl peroxide,cumene hydroperoxide, tertiary butyl hydroperoxide, and the like. Aboveall, from the viewpoints of ease of handling and being excellent instability, α,α′-azoisobutyronitrile is preferred.

Although the amount of an oil-soluble radical initiator used is notparticularly restricted, for example, it is desirable that the amountthereof be 0.00003 to 0.002 moles or so relative to 1 mole of(meth)acrylic acid. In the case where the amount of an oil-solubleradical initiator used is less than 0.00003 moles, there is a case wherethe process becomes uneconomical because the reaction rate becomes slow.Moreover, in the case where the amount of an oil-soluble radicalinitiator used is more than 0.002 moles, there is a case where heatremoval becomes difficult because the polymerization proceeds rapidlyand vigorously and the reaction becomes difficult to be controlled.

The concentration of an oil-soluble radical initiator (residualinitiator) remaining in the binder for lithium ion secondary batterypositive electrodes of the present invention is 500 ppm or less,preferably 300 ppm or less, and further preferably 200 ppm or less. Thatis, the proportion of an oil-soluble radical initiator included in thebinder to a binder resin included in the binder is 500 ppm or less or soin terms of the mass ratio. By making the remaining amount of theinitiator 500 ppm or less, a rise in internal resistance of a batterycan be suppressed and the high-rate discharge characteristics of thebattery can be enhanced.

A method of reducing the residual initiator is not particularlyrestricted, examples thereof include a method of reducing the amount ofan oil-soluble radical initiator used, a method of excessively heating,a method of adding a reaction terminator, a method of washing with asolvent compatible with an oil-soluble radical initiator, and the like,and preferred is a method of washing with a solvent compatible with anoil-soluble radical initiator described below. Moreover, one of thesemethods may be carried out and two or more methods thereof may becombined to be carried out.

Although the reaction temperature is not particularly restricted, thereaction temperature is preferably 50 to 90° C. or so and morepreferably 55 to 75° C. or so. In the case where the reactiontemperature is less than 50° C., there is a case where the viscosity ofa reaction solution is increased and the reaction solution fails to beuniformly stirred. Moreover, in the case where the reaction temperatureis more than 90° C., the reaction proceeds rapidly and vigorously andthe reaction fails to be controlled. Although the reaction time cannotbe decided sweepingly since the period of time varies depending on thereaction temperature, the reaction time is usually 0.5 to 5 hours or so.

After the completion of the reaction, centrifugal filtration isperformed to remove the filtrate, a fresh solvent is added again to thesolid matter to be stirred, and centrifugal filtration is performed toremove the filtrate. With this setup, the residual initiator containedin a resin can be reduced.

For example, a reaction solution can be heated to 80 to 130° C. or soand the solvent can be distilled off to obtain an alkyl-modifiedcarboxyl group-containing interpolymer. In the case where the heatingtemperature is less than 80° C., there is a case where a long period oftime is required for drying. Moreover, in the case where the heatingtemperature is more than 130° C., there is a case where the solubilityof the resulting alkyl-modified carboxyl group-containing interpolymerto a liquid medium such as water is deteriorated.

Although the volume average particle diameter of the alkyl-modifiedcarboxyl group-containing interpolymer thus obtained is not particularlyrestricted, the volume average particle diameter thereof is preferably0.1 to 50 μm or so, more preferably 0.5 to 30 μm or so, and furtherpreferably 1 to 20 μm or so. When the volume average particle diameteris less than 0.1 μm, the amount of the binder required for sufficientlybinding the active material in the electrode is increased, and as aresult, there is a case where the rate characteristics are loweredbecause the surface of the active material is covered with the binder.Conversely, when the volume average particle diameter of theinterpolymer is more than 50 μm, there is a possibility that theresistance becomes large because the conductive additive is ununiformlydispersed. In this connection, these particles can be aggregated by theaddition of water or the like, and in this case, the volume averageparticle diameter may lie within the range of 100 to 1000 μm. In thisconnection, the volume average particle diameter of the alkyl-modifiedcarboxyl group-containing interpolymer refers to a value obtained bybeing measured by using normal hexane as a dispersant with the use of alaser diffraction type particle size distribution measuring apparatus(SALD-7100 available from SHIMADZU CORPORATION).

In the case where a binder composed of an alkyl-modified carboxylgroup-containing interpolymer of the present invention is used in anelectrode, the binder is usually dissolved or dispersed in a liquidmedium such as water described below to be used.

<Positive Electrode>

As mentioned above, by using the binder for lithium ion secondarybattery positive electrodes of the present invention in a positiveelectrode, excellent high-rate discharge characteristics can be impartedto a positive electrode for lithium ion secondary batteries.

For example, a positive electrode in the present invention is producedin the following manner. A positive electrode active material, aconductive additive, a binder of the present invention, and a liquidmedium such as water are mixed to prepare pasty slurry as a positiveelectrode mixture. The positive electrode mixture can be applied onto apositive electrode current collector to prepare a binder for lithium ionsecondary battery positive electrodes of the present invention. A binderof the present invention may be previously dissolved in a liquid mediumto be used or a powdery binder of the present invention and a positiveelectrode active material are previously mixed, after which the mixturemay be added with a liquid medium to be used.

(Positive Electrode Active Material)

The positive electrode active material is not particularly restrictedand a known positive electrode active material used in a lithium ionsecondary battery can be used. Specific examples of the positiveelectrode active material include lithium iron phosphate (LiFePO₄),lithium manganese phosphate (LiMnPO₄), lithium cobalt phosphate(LiCoPO₄), lithium iron pyrophosphate (Li₂FeP₂O₇), lithium cobaltcomposite oxide (LiCoO₂), spinel type lithium manganese composite oxide(LiMn₂O₄), lithium manganese composite oxide (LiMnO₂), lithium nickelcomposite oxide (LiNiO₂), lithium niobium composite oxide (LiNbO₂),lithium iron composite oxide (LiFeO₂), lithium magnesium composite oxide(LiMgO₂), lithium calcium composite oxide (LiCaO₂), lithium coppercomposite oxide (LiCuO₂), lithium zinc composite oxide (LiZnO₂), lithiummolybdenum composite oxide (LiMoO₂), lithium tantalum composite oxide(LiTaO₂), lithium tungsten composite oxide (LiWO₂),lithium-nickel-cobalt-aluminum composite oxide(LiNi_(0.8)Co_(0.15)Al_(0.05)O₂), lithium-nickel-cobalt-manganesecomposite oxide (LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂), a lithium-richternary nickel-cobalt-manganese composite oxide, manganese oxide-nickel(LiNi_(0.5)Mn_(1.5)O₄), manganese oxide (MnO₂), a vanadium-based oxide,a sulfur-based oxide, a silicate-based oxide, and the like. One kind ofthe positive electrode active material may be used alone and two or morekinds thereof may be used in combination.

(Conductive Additive)

Although the conductive additive is not particularly restricted as longas the additive has a conductivity, carbon powder is preferred. Examplesof the carbon powder include ones usually used, namely, carbon materialssuch as acetylene black (AB), Ketjen black (KB), graphite, carbonfibers, carbon tubes, graphene, amorphous carbon, hard carbon, softcarbon, glassy carbon, carbon nanofibers, and carbon nanotubes. One kindof the conductive additive may be used alone and two or more kindsthereof may be used in combination.

As the conductive additive, of these, from the viewpoint of enhancementin conductivity, carbon nanofibers and carbon nanotubes are preferredand carbon nanotubes are more preferred. In the case where carbonnanotubes are used as the conductive additive, although the amount ofthe nanotubes used is not particularly restricted, for example, thenanotubes in an amount of preferably 30 to 100% by mass or so, morepreferably 40 to 100% by mass or so, are used relative to the wholeconductive additive. When the amount of the carbon nanotubes used isless than 30% by mass, there is a case where a sufficient conductivepathway is not secured between a positive electrode active material anda positive electrode current collector, and in particular, a sufficientconductive pathway fails to be formed at the time of high-speed chargingand discharging. In this connection, a carbon nanofiber refers to afibrous material with a thickness of several nm to several hundreds ofnm, and in particular, a fibrous material having a hollow structure isreferred to as a carbon nanotube. There are various kinds of carbonnanotubes such as a single-walled carbon nanotube and a multi-walledcarbon nanotube. Although these are produced by various methods such asa vapor phase growth method, an arc-discharge method, and a laserevaporation method, the production method is not particularlyrestricted.

Although the amount of the conductive additive used in a positiveelectrode is not particularly restricted, for example, in the case wherethe total of the positive electrode active material, the conductiveadditive, and the binder is defined as 100% by mass, the conductiveadditive in an amount of preferably 1.5 to 20% by mass or so, morepreferably 2.0 to 10% by mass or so, is used. In this connection, whenthe amount of the conductive additive used is less than 1.5% by mass,there is a case where the conductivity of a positive electrode fails tobe sufficiently enhanced. Moreover, when the amount of the conductiveadditive used is more than 20% by mass, the positive electrode is notpreferred in the point that high capacity is hardly attained at the timeof charging and discharging of a battery because the proportion of thepositive electrode active material is relatively reduced, aggregation ofthe positive electrode active material is caused because carbon powderas the conductive additive repels water and becomes difficult to beuniformly dispersed in the case of using water as the liquid medium, andthe amount of a binder used is increased because the conductive additiveis small and has a large surface area as compared with the positiveelectrode active material, and the like.

Although the amount of the binder of the present invention used in apositive electrode is not particularly restricted, for example, in thecase where the total of the positive electrode active material, theconductive additive, and the binder is defined as 100% by mass, thebinder in an amount of preferably 0.5% by mass or more and 30% by massor less, more preferably 1% by mass or more and 20% by mass or less andfurther preferably 2% by mass or more and 8% by mass or less, is used.When the amount of the binder is too large, there is a case where apositive electrode is made larger in resistance inside the electrode anddeterioration in high-rate discharge characteristics is caused.Moreover, when the amount of the binder is too small, there is a casewhere charge-discharge cycle characteristics are lowered.

(Liquid Medium)

Examples of the liquid medium include water and a non-aqueous medium. Asthe non-aqueous medium, aliphatic hydrocarbons such as n-octane,isooctane, nonane, decane, decalin, pinene, and chlorododecane;cycloaliphatic hydrocarbons such as cyclopentane, cyclohexane,cycloheptane, and methylcyclopentane; aromatic hydrocarbons such asstyrene, chlorobenzene, chlorotoluene, ethylbenzene, diisopropylbenzene,and cumene; alcohols such as methanol, ethanol, propanol, isopropanol,butanol, benzyl alcohol, and glycerine; ketones such as acetone, methylethyl ketone, cyclopentanone, and isophorone; ethers such as methylethyl ether, diethyl ether, tetrahydrofuran, and dioxane; lactones suchas γ-butyrolactone and δ-butyrolactone; lactams such as β-lactam;chain/cyclic amides such as dimethylformamide, N-methylpyrrolidone, anddimethylacetamide; nitrile group-containing compounds such as methylenecyanohydrin, ethylene cyanohydrin, 3,3′-thiodipropionitrile, andacetonitrile; nitrogen-containing heterocyclic compounds such aspyridine and pyrrole; glycols such as ethylene glycol and propyleneglycol; diethylene glycols such as diethylene glycol, diethylene glycolmonoethyl ether, and diethylene glycol ethyl butyl ether; esters such asethyl formate, ethyl lactate, propyl lactate, methyl benzoate, methylacetate, and methyl acrylate, and the like are exemplified. Moreover, asthe non-aqueous medium, mixtures such as lacquer, gasoline, naphtha, andkerosene can be used. Of the above-mentioned liquid mediums, water ispreferred from the viewpoints of solubility and economy and it ispreferred that, with the use of an alkaline component such as sodiumhydroxide, the pH of a solution be adjusted to 6 to 8 to be used.

In the case where a binder containing an alkyl-modified carboxylgroup-containing interpolymer of the present invention is dissolved ordispersed in a liquid medium to be used, the content of the interpolymerin the whole dissolving liquid or dispersing liquid is preferably 0.2 to70% by mass or so, more preferably 0.5 to 60% by mass or so, furtherpreferably 0.5 to 50% by mass or so, and especially preferably 2 to 35%by mass or so.

The pH of the slurry is preferably 4 to 10, more preferably 5 to 9, andfurther preferably 6 to 8. When the pH becomes 4 or less, there is afear that battery performance is lowered due to the corrosion of apositive electrode current collector and the degradation of anelectrolytic solution or a positive electrode active material. Moreover,when the pH becomes 10 or more, there is a fear that battery performanceis lowered because a positive electrode current collector constituted ofmetal such as aluminum is corroded.

For the purpose of adjusting the pH of slurry, a pH adjusting agent maybe used. Examples of the pH adjusting agent include an acidic pHadjuster and an alkaline pH adjuster. Examples of the acidic pH adjusterinclude inorganic acids such as hydrochloric acid, sulfuric acid,phosphoric acid, and nitric acid; and organic acids such as formic acid,acetic acid, propionic acid, and citric acid. Moreover, examples of thealkaline pH adjuster include inorganic alkalis such as sodium hydroxide,potassium hydroxide, and lithium hydroxide; organic alkalis such asammonia, methylamine, and ethylamine, and the like.

Moreover, in the present invention, in order to enhance the coatingproperties of slurry and enhance the charge-discharge characteristics,an additive may be used. Examples of the additive includecellulose-based polymers such as carboxymethyl cellulose, methylcellulose, and hydroxypropyl cellulose, polyacrylates such as sodiumpolyacrylate, polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, a (meth)acrylic acid-vinyl alcohol copolymer, a maleicacid-vinyl alcohol copolymer, a modified polyvinyl alcohol, polyethyleneglycol, an ethylene-vinyl alcohol copolymer, a polyvinyl acetatepartially ketonized product, and the like. One kind of the additive maybe used alone and two or more kinds thereof may be used in combination.

Although the proportion of these additives used is not particularlyrestricted, the proportion thereof is preferably less than 300 parts bymass, more preferably 30 parts by mass or more and 250 parts by mass orless, and further preferably 40 parts by mass or more and 200 parts bymass or less relative to 100 parts by mass of an alkyl-modified carboxylgroup-containing interpolymer constituting the binder. When theproportion thereof lies within such a range, an electrode excellent insmoothness can be obtained. In this connection, such an additive may beadded to a binder to be used and may be added to the above-mentionedslurry to be used.

In this connection, in a positive electrode, without impairing theobject of the present invention, water-soluble compounds such as acrylicacid, an acrylic acid metal neutralized salt, methacrylic acid, amethacrylic acid metal neutralized salt, carboxymethyl cellulose, andhydroxyethyl cellulose and conventional binders such asstyrene/butadiene copolymer-containing emulsion, butadiene/acrylonitrilecopolymer-containing emulsion, PVdF-containing emulsion, andpolytetrafluoroethane polymer-containing emulsion may be used togetherwith the binder of the present invention.

(Positive Electrode Current Collector)

The material for the positive electrode current collector is notparticularly restricted as long as the material has an electronconductivity and can make a current pass through a positive electrodematerial provided thereon. Examples of the material for the positiveelectrode current collector include conductive substances made of C, Ti,Cr, Mo, Ru, Rh, Ta, W, Os, Ir, Pt, Au, Al, and the like, an alloycontaining two or more kinds of these conductive substances (forexample, stainless steel), and the like. From the viewpoints of beinghigh in electrical conductivity and being satisfactory in stability inan electrolytic solution and oxidation resistance, as the material forthe positive electrode current collector, C, Al, stainless steel, andthe like are preferred, and from the viewpoint of material costs, Al orthe like is further preferred.

The shape of the positive electrode current collector is notparticularly restricted and examples thereof include a foil-like shape,a three-dimensional shape, and the like. In this connection, even in thecase of a binder being low in adhesion to a positive electrode currentcollector, when the positive electrode current collector has athree-dimensional shape (sponged metal, mesh, woven fabric, nonwovenfabric, expanded shape, or the like), an electrode with a high capacitydensity is obtained and high-rate charge and discharge characteristicsthereof also become satisfactory.

In this connection, even in the case of a foil-shaped positive electrodecurrent collector, a primer layer can be previously formed on thecurrent collector surface to attain highly enhanced capacity. As theprimer layer, ones that are satisfactory in adhesion to a positiveelectrode active material layer and a positive electrode currentcollector and have a conductivity can be used. For example, a binderwith which a carbon-based conductive additive is mixed can be appliedonto a positive electrode current collector so that the coating layerhas a thickness of 0.1 μm to 50 μm to be formed into a primer layer.

As a conductive additive used for the primer layer, carbon powder ispreferred. When a metal-based conductive additive is adopted, thecapacity density can be heightened, but there is a case where theinput-output characteristics become poor. When a carbon-based one isadopted as the conductive additive, the input-output characteristicsbecome easy to be enhanced. Examples of the carbon-based conductiveadditive include KB, AB, VGCF, graphite, graphene, carbon tubes, and thelike. One kind of the conductive additive may be used alone and two ormore kinds thereof may be used in combination. As the conductiveadditive used for the primer layer, from the viewpoints of conductivityand costs, KB or AB is preferred.

As a binder used for the primer layer, any kind of binder is acceptableas long as the binder can bind the carbon-based conductive additive.However, when the binder of the present invention or a water-basedbinder such as PVA, CMC, or sodium alginate is used to form a primerlayer, there is a case where the primer layer is dissolved at the timeof forming an active material layer and effects are not exertedsignificantly. On that account, at the time of using such a water-basedbinder, it is good for the primer layer to be previously crosslinked.Examples of a crosslinking material include a zirconia compound, a boroncompound, a titanium compound, and the like and it is good for thecrosslinking material in an amount of 0.1 to 20% by mass or so relativeto the amount of the binder to be added to slurry to be formed into aprimer layer. By virtue of the primer layer thus prepared, in afoil-shaped positive electrode current collector, the capacity densitycan be enhanced with the use of a water-based binder. Furthermore,high-rate charge and discharge characteristics become satisfactorybecause polarization is made small even when charging and dischargingare performed under a high current condition. In this connection, theprimer layer creates the above-mentioned effect in the case of afoil-shaped positive electrode current collector. The same holds truefor the case of a three-dimensionally shaped positive electrode currentcollector.

For example, a positive electrode for the lithium ion secondary batteryof the present invention may be a positive electrode for lithium ionsecondary batteries in which a positive electrode active materialprovided with a metal oxide represented by the following CompositionFormula 1 on its active material particle surface and the binder of thepresent invention are used.LiαMβOγ  Composition Formula 1

In Composition Formula 1, M represents at least one kind of metalelement selected from the group consisting of Al, Ti, Cr, Mn, Fe, Co,Ni, Cu, Zr, Nb, Mo, Ag, Ta, W, and Ir, and α, β, and γ satisfy theequations of 0≤α≤6, 1≤β≤5, and 0<γ≤12, respectively. Among these, fromthe viewpoint of heat resistance, it is preferred that M represent Zr.

In this connection, in the present specification, “a positive electrodeactive material provided with a metal oxide on its active materialparticle surface” refers to a concept that includes a positive electrodeprovided with an overcoat layer constituted of a metal oxide on itselectrode surface, a positive electrode active material covered with ametal oxide on its particle surface, and using both of the positiveelectrode and the positive electrode active material in combination.

By providing the active material particle surface of a positiveelectrode active material with a metal oxide, matters of concern at thetime of using a water-based binder like the present invention, namely,lowering in capacity of the positive electrode active material caused byelution of lithium from the positive electrode active material andoxidative decomposition of the water-based binder at the time ofcharging can be prevented and the high-rate discharge characteristicscan be further enhanced.

Furthermore, by providing the active material particle surface with ametal oxide, such a positive electrode active material having anoperating voltage of more than 4 V can be used in a conventionalelectrolytic solution. For example, although there is a fear thatelectrons are taken out from an electrolytic solution and oxidativedecomposition is caused because the redox potential attributed to avalence change from divalent one to tetravalent one or from tetravalentone to divalent one of a phosphoric acid transition metal lithiumcompound, in which the transition metal is constituted of Ni or Co, isvery high, by providing the active material particle surface with anoxidation-resistant lithium transition metal oxide, the positiveelectrode active material can be prevented from being brought intodirect contact with an electrolytic solution.

By providing the electrode surface of a positive electrode with anovercoat layer constituted of a metal oxide and covering the activematerial particle surface with a metal oxide, the effect is exerted at ahigher level.

A method of covering the active material particle surface with a metaloxide is not particularly restricted and a conventionally performedmethod such as an immersion method in which a prescribed amount of acoating liquid containing a metal oxide is added with a prescribedamount of active material powder to be mixed therewith can be used.Examples of a more convenient method include a method of spraying metaloxide microparticles onto active material particles with the use of asprayer. According to this method, the active material particle surfacecan be suitably covered with a metal oxide. A coating method by asprayer can be easily performed and is also advantageous from an aspectof cost. The same method can be used also in the case where an electrodesurface is coated with a metal oxide.

In the case where the electrode surface of a positive electrode isprovided with an overcoat layer constituted of a metal oxide, it ispreferred that the thickness of the overcoat layer constituted of ametal oxide on the electrode surface be 0.1 to 10 μm or so. When thethickness is less than 0.1 μm, there is a case where lowering incapacity of the positive electrode active material and oxidativedecomposition of the water-based binder at the time of charging fail tobe sufficiently prevented. Moreover, when the thickness is more than 10μm, the electrode thickness is increased and there is a tendency that abattery deteriorates in the high-rate discharge characteristics since itis necessary for the impedance of the battery to be enhanced as well asbeing made low in capacity.

A positive electrode active material can be provided with a mixture of ametal oxide and a conductive additive on its particle surface. In thiscase, for example, a method of previously providing the particle surfacewith a mixture of a metal oxide and a carbon precursor and carbonizingthe carbon precursor by a heating treatment method may be adopted. Inthis connection, the heating treatment method refers to a method ofsubjecting a carbon precursor to a heating treatment at 600 to 4,000° C.or so in a non-oxidizing atmosphere (hardly oxidizable condition such asa reducing atmosphere, an inert atmosphere, and a pressure-reducedatmosphere) to carbonize the carbon precursor and making the resultingcarbon material exhibit its conductivity.

The carbon precursor is not particularly restricted as long as theprecursor can be formed into a carbon material by a heating treatmentand examples thereof include glucose, citric acid, pitch, tar, bindermaterials used for electrodes, and the like.

In the case where the total of the metal oxide and the carbon precursoris defined as 100% by mass, it is preferred that the proportion of thecarbon precursor be 0.5 to 20% by mass or so. When the proportion of thecarbon precursor is less than 0.5% by mass, there is a case where theconductivity of a positive electrode fails to be sufficiently enhanced.Moreover, when the proportion of the carbon precursor is more than 20%by mass, there is a tendency that a possibility of causing aggregationof the positive electrode active material is increased because carbonrepels water and becomes difficult to be uniformly dispersed at the timeof preparing water-based slurry. In the case where the positiveelectrode active material is so-called carbon-coated powder or acarbon-based conductive additive is used as the positive electrodeactive material, a possibility of causing aggregation of the positiveelectrode active material is increased because carbon repels water andthe positive electrode active material becomes difficult to be uniformlydispersed in water-based slurry at the time of preparing the slurry. Inthat case, it is preferred that a surfactant be added to the slurry. Asaponin, a phospholipid, a peptide, Triton, and the like are effectiveas a surfactant and the surfactant in an amount of 0.01 to 0.1% by massor so relative to the whole slurry needs only to be added thereto.

<Battery>

The positive electrode for lithium ion secondary batteries of thepresent invention can be used to prepare the lithium ion secondarybattery of the present invention.

As a negative electrode, materials usually used for lithium ionsecondary batteries can be used. For example, the material needs only tobe constituted of at least one or more kinds of element selected fromthe group consisting of Li, Na, C, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr,Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Nb, Mo, Pd, Ag, Cd, In, Sn, Sb,W, Pb, and Bi, an alloy composed of these elements, an oxide composedthereof, a chalcogenide composed thereof, or a halide composed thereof.

Of these, from the viewpoint that a discharge plateau region can beobserved within the range of 0 to 1 V (against the lithium potential),preferred is at least one or more kinds of element selected from thegroup consisting of Li, C, Mg, Al, Si, Ti, Zn, Ge, Ag, Cu, In, Sn, andPb, an alloy composed of these elements, or an oxide composed thereof.Furthermore, from the viewpoint of the energy density, Al, Si, Zn, Ge,Ag, Sn, or the like is preferred as the element, the respectivecombinations of Si—Al, Al—Zn, Si—Mg, Al—Ge, Si—Ge, Si—Ag, Zn—Sn, Ge—Ag,Ge—Sn, Ge—Sb, Ag—Sn, Ag—Ge, Sn—Sb, and the like, and the like arepreferred as the alloy, SiO, SnO, SnO₂, CuO, Li₄Ti₅O₁₂ or the like ispreferred as the oxide.

Among these, it is more preferred that Si-based materials be used, sincethe high-rate discharge characteristics, as well as the energy density,can be enhanced. However, in many types of Si-based materials, the cyclecharacteristics thereof are not sufficiently exerted because theSi-based material varies greatly in its volume due to charging anddischarging. On that account, it is preferred that SiO, which isdecomposed, in an initial charging process, into two components of asolid electrolyte having a lithium ion conductivity and a materialcapable of reversibly occluding/releasing lithium, be used.

In this connection, it does not matter at all if two or more kinds ofthese materials capable of reversibly occluding/releasing lithium areused.

Moreover, a lithium salt is preferred as an electrolyte salt because itis necessary for a lithium ion secondary battery prepared with thepositive electrode of the present invention to contain lithium ions.Although the lithium salt is not particularly restricted, specificexamples thereof include lithium hexafluorophosphate, lithiumperchlorate, lithium tetrafluoroborate, trifluoromethane sulfonic acidimide lithium, and the like. One kind of these lithium salts may be usedalone or two or more kinds thereof may be mixedly used. Since theabove-mentioned lithium salts are high in electronegativity and easilyionized, a secondary battery can be made excellent in charge-dischargecycle characteristics and the charge-discharge capacity of the secondarybattery can be enhanced.

As a solvent for the electrolyte, for example, propylene carbonate,ethylene carbonate, dimethyl carbonate, diethyl carbonate,γ-butyrolactone, and the like can be used, and one kind of thesesolvents may be used alone or two or more kinds thereof may be mixedlyused. In particular, propylene carbonate as a simple substance, amixture of ethylene carbonate and diethyl carbonate, or γ-butyrolactoneas a simple substance is suitable. In this connection, the mixing ratioof ethylene carbonate and diethyl carbonate in the mixture can bearbitrarily adjusted within a range where the amount of one component is10% by volume or more and 90% by volume or less.

Moreover, the electrolyte for the lithium ion secondary battery of thepresent invention may be a solid electrolyte or an ionic liquid.According to the lithium ion secondary battery of the above-describedstructure, a lithium ion secondary battery can be made to function as alithium ion secondary battery excellent in high-rate dischargecharacteristics.

Although the structure of the lithium ion secondary battery is notparticularly limited, the present invention can be applied to anexisting battery form/structure of a battery such as a laminated batteryor a wound battery.

<Electrical Equipment>

Since a lithium ion secondary battery provided with the positiveelectrode of the present invention is satisfactory in safety, thebattery can be utilized as a power source for various kinds ofelectrical equipment (including a vehicle that uses electricity).

Examples of the electrical equipment include an air-conditioner, awashing machine, a television receiver, a refrigerator, a freezer,cooling equipment, a notebook personal computer, a tablet computer, asmartphone, a personal computer keyboard, a display for a personalcomputer, a desktop type personal computer, a CRT monitor, a printer, anall-in-one personal computer, a mouse, a hard disk, personal computerperipheral equipment, an iron, a clothes dryer, a window fan, atransceiver, an air blower, a ventilating fan, a television receiver, amusic recorder, a music player, an oven, a kitchen range, a toilet seatwith a washing function, a warm air heater, a car compo, car navigationequipment, a flashlight, a humidifier, portable Karaoke equipment, aventilating fan, a dryer, an air cleaner, a cellular phone, an emergencylight, a game machine, a sphygmomanometer, a coffee mill, a coffeemaker, a kotatsu heater, a copying machine, a disk changer, a radio, ashaver, a juicer, a shredder, a water purifier, a lighting apparatus, adehumidifier, a dish dryer, a rice cooker, a stereophonic apparatus, astove, a speaker, a trouser press, a vacuum cleaner, a body fat meter, abody weight meter, a bathroom scale, a movie player, an electricallyheated carpet, an electric rice cooker, a desk lamp, an electric pot, anelectronic game machine, a portable game machine, an electronicdictionary, an electronic organizer, a microwave oven, anelectromagnetic cooker, an electronic calculator, an electric cart, anelectric wheelchair, an electric power tool, an electric toothbrush, anelectric foot warmer, electric hair clippers, a desk-telephone, a clock,an intercom, an air circulator, an electric bug killer, a hot plate, atoaster, a dryer, an electric drill, a water heater, a panel heater, acrusher, a soldering iron, a video camera, a video cassette recorder,facsimile equipment, a food processor, a futon dryer, a headphone, amicrophone, a massage machine, a sewing machine, a rice-cake makingmachine, a floor heating panel, a lantern, a remote controller, arefrigerating/warming cabinet, a water cooler, a cold air blower, a wordprocessor, an electric whisk, electronic musical instruments, amotorbike, electric toys, a lawn mower, an electric float, an electricbicycle, an automobile, a hybrid vehicle, a plug-in hybrid vehicle, anelectric vehicle, a railway vehicle, a ship, an airplane, a storagebattery for emergency, and the like.

EXAMPLES

Hereinafter, the present invention will be described in more detail byreference to examples, but the present invention should not be limitedby these examples at all.

<Preparation of Binder>

Example 1

Into a 500-mL four-necked flask equipped with a stirrer, a thermometer,a nitrogen inlet tube, and a condenser, 45 g (0.625 moles) of acrylicacid, 0.45 g of BLEMMER VMA70 (available from NOF CORPORATION, a mixtureof 10 to 20 parts by mass of stearyl methacrylate, 10 to 20 parts bymass of eicosanyl methacrylate, 59 to 80 parts by mass of behenylmethacrylate, and 1 part by mass or less of tetracosanyl methacrylate)as a (meth)acrylic acid alkyl ester having an alkyl group with 18 to 24carbon atoms, 150 g of normal hexane, and 0.081 g (0.00035 moles) of2,2′-azobismethylisobutylate were placed. Then, the contents werestirred and uniformly mixed, after which nitrogen gas was blown into thesolution in order to remove oxygen existing in the upper space of thereaction vessel, in the raw materials, and in the solvent. Then, under anitrogen atmosphere, the contents were made to undergo a reaction for 4hours while maintaining the temperature at 60 to 65° C. After thecompletion of the reaction, the contents were cooled and subjected tocentrifugal filtration to remove the filtrate. To a polymer obtained byremoving the filtrate, 100 g of normal hexane was added, the contentswere stirred and subjected to centrifugal filtration, and the filtratewas removed to wash the polymer. The obtained polymer was heated to 90°C., the remaining normal hexane was distilled off, and furthermore, thecontents were dried under reduced pressure for 8 hours at 110° C. and 10mmHg to obtain 43 g of a finely powdered white alkyl-modified carboxylgroup-containing interpolymer (a). The obtained alkyl-modified carboxylgroup-containing interpolymer (a) was evaluated for the amount of theremaining initiator in the following manner. Results are shown in Table1.

The amount of the remaining initiator was measured by gas chromatography(Column: capillary column Rtx-200, 30 m in length×0.53 mm in innerdiameter, available from SHIMADZU CORPORATION, Column temperature: 160°C., Detector: FID).

Comparative Example 1

Into a 500-mL four-necked flask equipped with a stirrer, a thermometer,a nitrogen inlet tube, and a condenser, 45 g (0.625 moles) of acrylicacid, 0.45 g of BLEMMER VMA70 (available from NOF CORPORATION, a mixtureof 10 to 20 parts by mass of stearyl methacrylate, 10 to 20 parts bymass of eicosanyl methacrylate, 59 to 80 parts by mass of behenylmethacrylate, and 1 part by mass or less of tetracosanyl methacrylate)as a (meth)acrylic acid alkyl ester having an alkyl group with 18 to 24carbon atoms, 150 g of normal hexane, and 0.081 g (0.00035 moles) of2,2′-azobismethylisobutylate as an initiator were placed. Then, thecontents were stirred and uniformly mixed, after which nitrogen gas wasblown into the solution in order to remove oxygen existing in the upperspace of the reaction vessel, in the raw materials, and in the solvent.Then, under a nitrogen atmosphere, the contents were made to undergo areaction for 4 hours while maintaining the temperature at 60 to 65° C.After the completion of the reaction, the slurry produced was heated to70° C. and the normal hexane was distilled off to obtain 42 g of afinely powdered white alkyl-modified carboxyl group-containinginterpolymer (b). The obtained alkyl-modified carboxyl group-containinginterpolymer (b) was evaluated for the amount of the remaining initiatorin the same manner as that in Example 1. Results are shown in Table 1.

TABLE 1 Remaining Amount of Initiator Binder (ppm) Example 1Interpolymer (a) 200 Comparative Interpolymer (b) 1000 Example 1<Preparation of LiFePO₄ Positive Electrode>

Example 2

In water, 1 g of the alkyl-modified carboxyl group-containinginterpolymer (a) obtained in Example 1 was dissolved and the pH of thesolution was adjusted to 6 to 8 with a 6% by mass aqueous sodiumhydroxide solution to prepare a 10% by mass aqueous binder solution. Toa mixture of 60 parts by mass of the obtained aqueous binder solution,90 parts by mass of lithium iron phosphate as a positive electrodeactive material, 2 parts by mass of carbon nanotubes as a conductiveadditive, and 2 parts by mass of Ketjen black, water was added and thecontents were stirred to prepare a slurried positive electrode mixturewith a solid content concentration of 40% by mass. The mixture wasapplied onto a sheet of aluminum foil with a thickness of 20 μm anddried, after which, by the use of a roll press machine (available fromOono-roll Corporation), the aluminum foil and the coating film weretightly bonded together and then subjected to a heating treatment (underreduced pressure, 180° C., 3 hours or more) to prepare a positiveelectrode for testing. The composition of the positive electrode fortesting was shown in Table 2. The positive electrode capacity density ofthe positive electrode for testing was set to 0.7 mAh/cm² (averagethickness of active material layer: 35 μm).

Comparative Example 2

A positive electrode for comparison was prepared to be evaluated in thesame manner as that in Example 2 except that an alkyl-modified carboxylgroup-containing interpolymer (b) was used instead of the alkyl-modifiedcarboxyl group-containing interpolymer (a) in Example 2. The compositionof the positive electrode for comparison was shown in Table 2.

TABLE 2 Composition Ratio Active Conductive Conductive of PositiveElectrode Material Binder Additive Additive (% by mass) A B C D A:B:C:DExample 2 LFP Interpolymer (a) CNT KB 90:6:2:2 Comparative LFPInterpolymer (b) CNT KB 90:6:2:2 Example 2

In Table 2, LFP means lithium iron phosphate, CNT means a carbonnanotube, and KB means Ketjen black.

Assembly of Battery

Example 3

A coin cell (CR2032) which is provided with a positive electrode fortesting obtained in Example 2; a counter electrode constituted ofmetallic lithium; a glass filter (GA-100 available from Advantec ToyoKaisha, Ltd.) as a separator; and a solution, as an electrolyticsolution, prepared by dissolving LiPF₆ in a concentration of 1 mol/L ina solvent prepared by mixing ethylene carbonate (EC) and diethylcarbonate (DEC) at a volume ratio of 1:1 and by being added with 1% bymass of vinylene carbonate (VC) as an additive for electrolyte, wasprepared to be subjected to an aging treatment in which a cycle test isperformed two times at 0.2 C under an environment of 30° C.

High-Rate Discharge Test

With regard to a coin cell of Example 3, a high-rate discharge test wasperformed under an environment of 30° C. With regard to the condition ofthe high-rate discharge test, the coin cell was charged at 0.5 C anddischarging was performed at each of the respective rates of 0.5 C, 1 C,3 C, 5 C, 10 C, and 30 C. In this connection, the cut-off potential wasset to 4.2-2.0 V (vs. Li⁺/Li). Active material capacities at therespective discharge rates obtained as the result of the high-ratedischarge test are shown in Table 3. Average potentials (V vs. Li⁺/Li)at the respective discharge rates obtained as the result of thehigh-rate discharge test are shown in Table 4. In the case where theactive material capacity is 0 mAh/g, a value of the average potential isshown as “−” because the average potential during discharging fails tobe measured.

Comparative Example 3

A battery was assembled in the same manner as that in Example 3 exceptthat a positive electrode for comparison obtained in Comparative Example2 was used and a high-rate discharge test was carried out. Results areshown in Tables 3 and 4.

TABLE 3 Active Material Capacities at Respective Discharge Rates (mAh/g)Example 0.2 C. 0.5 C. 1 C. 3 C. 5 C. 10 C. 30 C. Example 2 166 163 162154 148 133 52 Comparative 158 142 126 56 15 0 0 Example 2

TABLE 4 Average Potentials during Discharging at Respective DischargeRates (V) Example 0.2 C. 0.5 C. 1 C. 3 C. 5 C. 10 C. 30 C. Example 23.35 3.33 3.32 3.25 3.2 3.07 2.87 Comparative 3.35 3.33 3.30 2.95 2.73 —— Example 2

In general, a battery shows a tendency that the internal resistancebecomes larger as the rate is made higher and the active materialcapacity and the average potential are decreased, but as shown in Table3 and Table 4, it has become apparent that, in the case of using thepositive electrode of Example 2, high discharge capacity and highdischarge potential are attained even at a high rate of 30 C.

The invention claimed is:
 1. A binder for lithium ion secondary battery positive electrodes, comprising: a binder resin; and an oil-soluble radical initiator in a concentration of 500 ppm or less, wherein the binder resin comprises an alkyl-modified carboxyl group-containing interpolymer prepared by copolymerizing a (meth)acrylic acid alkyl ester having an alkyl group with 18 to 24 carbon atoms in a proportion of 0.1 to 10 parts by mass with 100 parts by mass of (meth)acrylic acid.
 2. The binder for lithium ion secondary battery positive electrodes according to claim 1, wherein the oil-soluble radical initiator is at least one kind selected from the group consisting of an organic peroxide, an azo compound, and a redox initiator.
 3. The binder for lithium ion secondary battery positive electrodes according to claim 1, wherein the binder resin is a suspension polymer, an emulsion polymer, a dispersion polymer, or a precipitation polymer constituted of monomer units.
 4. A positive electrode for lithium ion secondary batteries, comprising: an active material; a conductive additive; and the binder for lithium ion secondary battery positive electrodes according to claim
 1. 5. The positive electrode for lithium ion secondary batteries according to claim 4, wherein the binder in an amount of 0.5 to 30% by mass relative to the total mass of the active material, the conductive additive, and the binder is included.
 6. A lithium ion secondary battery, being provided with the positive electrode for lithium ion secondary batteries according to claim
 4. 7. Electrical equipment, being mounted with the lithium ion secondary battery according to claim
 6. 8. A method of binding an active material and a conductive additive to a current collector, the method comprising applying to the current collector the binder according to claim 1, the active material and the conductive additive, wherein the method produces a lithium ion secondary battery positive electrode.
 9. A method of producing a binder for lithium ion secondary battery positive electrodes, comprising the step of mixing a binder resin comprising an alkyl-modified carboxyl group-containing interpolymer prepared by copolymerizing a (meth)acrylic acid alkyl ester having an alkyl group with 18 to 24 carbon atoms in a proportion of 0.1 to 10 parts by mass with 100 parts by mass of (meth)acrylic acid and an oil-soluble radical initiator in a concentration of 500 ppm or less. 